ubifs: Rework ubifs_assert()

With having access to struct ubifs_info in ubifs_assert() we can
give more information when an assert is failing.
By using ubifs_err() we can tell which UBIFS instance failed.

Also multiple actions can be taken now.
We support:
 - report: This is what UBIFS did so far, just report the failure and go
   on.
 - read-only: Switch to read-only mode.
 - panic: shoot the kernel in the head.

Signed-off-by: Richard Weinberger <richard@nod.at>